1. Field of the Invention
This invention relates to an ink to be used in an ink jet recording method in which ink is ejected from a recording head through action of thermal energy, an ink cartridge, an ink jet recording method and an ink jet recording apparatus using such an ink. More particularly, the present invention relates to an ink containing a coloring material having a specific structure that can improve the ink ejection stability and shows a high storage stability and a high color developability as well as an ink cartridge, an ink jet recording method and an ink jet recording apparatus using such an ink.
2. Related Background Art
Ink jet recording is a recording method in which ink is applied to a recording medium such as a sheet of paper to form an image thereon. It enables printing with a less expensive apparatus, a high resolution and a high speed. Ink jet recording has become very popular in recent years as means for forming color images with ease and there is an increasing demand for higher quality color images.
Currently, various pigments are being used as coloring materials to be used for ink jet recording. Since azo pigments particularly show an excellent coloring ability and are advantageous in terms of cost, various developments are under way.
For example, there is a proposal of synthesizing a pigment so as to allow two yellow monoazo pigments having different structures to coexist and using an ink containing the obtained pigment composition to improve the optical resistance and the thermal resistance (see, e.g., Japanese Patent Application Laid-Open No. H10-158555). There is a proposal of using a monoazo pigment with a purity of 90% or more to improve the storage stability of ink (see, e.g., Japanese Patent Application Laid-Open No. 2004-123866).
Various proposals have also been made for the purpose of improving the reliability (storage stability, ejection stability and so on) and the image quality (fixing ability, highlighter resistance and so on) of an ink jet recording ink prepared by using pigments as coloring materials. For example, there is a proposal of using an ink containing a compound having an alkylene oxide chain to improve the ejection performance and prevent the ink ejection micro-orifices arranged at a recording head from clogging (see, e.g., Japanese Patent Application Laid-Open No. H09-165538). Inks prepared so as to contain a 1,2-alkanediol or a glycol monoether for the purpose of improving the permeability of ink have also been proposed (see, e.g., Japanese Patent Application Laid-Open No. H07-157698 and Japanese Patent Application Laid-Open No. 2001-354888).
As described above, in particular, recently attention is paid to water-soluble organic solvents and additives contained in ink along with coloring materials, and then design is made for their properties and contents to solve the above-identified problems, which has become the mainstream.
The inventors of the present invention conducted evaluation of ejection stability of ink by loading an ink jet recording apparatus adapted to eject ink from the recording head thereof through action of thermal energy (to be also referred to as thermal type ink jet recording apparatus hereinafter) with an ink containing a monoazo pigment as coloring material. As a result, it was found that various phenomena occur as ink is ejected continuously, which phenomena include that the volume of the ejected ink gradually decreases and that impact position of ink on the recording medium becomes shifted. In short, the ejection stability is gradually degraded.
In view of the findings, the inventors of the present invention looked into the causes of degradation of the ejection stability and observed how the pigment dispersion that is contained in an ink changes as the ink passes through an ink flow path of the recording head and is ejected from the ejection orifices.
As a result, it was found that the dispersion breakdown of the pigment dispersion that is caused by the heat applied to it when ink droplets are ejected occurs rapidly and excessively particularly in the case of monoazo pigment dispersions that show a low thermal stability among numerous pigment dispersions.
In other words, the ejection stability is degraded not because of the clogged ink ejection orifices, which have hitherto been known. When heat is applied to an ink containing thermally unstable monoazo pigments as coloring material to raise the ink temperature, dispersion breakdown of the pigment dispersion takes place rapidly and excessively. Then, as a result, deposits adhere to the inside of the ink flow path and vicinity of the ejection orifices to consequently block the ink flow path of the recording head and degrade the ejection stability.
The above-described phenomenon has not been known so far and the present invention is based on this finding.
Thus, it is an object of the present invention to provide an ink to be used in an ink jet recording method in which ink is ejected from a recording head through action of thermal energy, which ink can improve the ink ejection stability and shows a high storage stability and a high color developability.
Another object of the present invention is to provide an ink set, an ink cartridge, an ink jet recording method and an ink jet recording apparatus using such an ink.